2G0 M. Humboldt on Isothermal Lines, and the 
M. Volney has endevoured to explain these phenomena by the 
frequency of the south-wqst winds, which drive back the warm 
air of the Gulf of Mexico towards these regions. A series of 
good observations, made for seven years by Colonel Mansfield 
at Cincinnati, on the banks of the Ohio, and recently publish- 
ed by Mr JJrake, in an excellent treatise on American meteoro- 
logy *, has removed the doubts which obscured this point. The 
thermometrical means prove that the isothermal lines do not 
rise again in the regions of the west. The quantity of heat 
which each point of the globe receives under the same parallels, 
is nearly equal on the east and the west of the Alleghany range, 
the winters being only a little milder to the west, and the sum- 
mers a little warmer f. The migrations of vegetables towards 
the north are favoured in the basin of the Mississippi, by the 
form and the direction of the valley which opens from the 
north to the south. In the Atlantic Provinces, on the contra- 
ry, the valleys are transverse, and oppose great obstacles to the 
passage of plants from one valley to another. 
If the isothermal lines remain parallel, or nearly so, to the 
equator, from the Atlantic shores of the New World to the east 
of the Mississippi and the Missouri, it cannot be doubted that 
they rise again beyond the Rochy Mountains, on the opposite 
coast of Asia, between 35° and 55° of latitude. To the consi- 
* Natural and Statistical View or Picture of Cincinnati and the Miami Coun- 
try ^ 1 vol. 8ro. Cincinnati. — H. See Warden’s Account of the United States f 
vol. ii. 236 for an abstract of Mr Drake’s, results. — Ed. 
The following comparison of t’nc mean temperatures has been deduced with 
great care. 
Cincinnati. 
Lat. 36° 6', west long. 84° 24^. 
Winter, 32°.9 Fahr. 
Spring, 54.1 
Summer, 72.9 
Autumn, .54.9 
Mean, 53.7 
Philadelphia. 
Lat. 39° 56', west long. 75° 16'. 
Winter, 32°. 2 Fahr. 
Spring, 51.4 
Summer, 73.9 
Autumn, 56.5 
Mean, 53.5 
I have taken for Philadelphia the means between the observations of Coxe and 
Rush. I have also referred for correction to the observations made by M. Legaux 
at Spiing-Mill upon the Schuylkill, to the north of Philadelphia, As Cincinnati 
is 512 feet above the level of the sea, its mean temperature is 1°.4 too low. - H. 
1 
